1. Field of Invention
This invention relates to a method and apparatus for modifying the behavior of fish. Specifically, it relates to a method and apparatus that modifies fish travel patterns by mechanically generating a current which creates a velocity head differential that fish can sense to guide them during their migration.
2. Description of Related Art
There is currently a conflict between the value of fish, especially with the decline of certain fish stocks, and the value of man-made devices that pose a risk to these fish. These devices may include hydroelectric power plants, municipal water intakes, thermal power plants, and irrigation systems.
Fish can be delayed when not properly directed to a bypass or when there is not sufficient current to guide them.
Fish can also suffer high mortalities when they come in contact with pumps or turbines, when they are subject to temperature variations induced by plants, or when they are pumped through irrigation systems and discharged onto fields.
Fish passage facilities, constructed to allow fish to navigate around hazards or obstacles, can be broken down into two general categories, those for upstream passage and those for downstream fish passage.
Upstream fish passage facilities typically incorporate some type of a structure to enable fish to swim up steep inclines. Traditional upstream fishways use a static head differential at the entrance of the fishway to create a current, attracting fish to the fishway. The static head differential is created by pumping water into a basin, where it then spills over a weir or through an orifice or slot. This generates a current field of limited influence, delaying the time fish are searching for the entrance.
Downstream fish passage systems consist of a barrier to keep fish out of the plant intake, and a physical structure to route them around the plant. Various physical devices and systems have been developed to prohibit fish entrance to or guide fish away from these potentially hazardous areas. These devices can be broken down into behavioral deterrents and physical barriers. Physical barriers work by limiting the opening size of the intake such that a certain fish species or life stage of fish can not enter. This includes barriers such as closely spaced bar racks, traveling screens, and fixed screens. The disadvantages of these structures are that they are difficult and expensive to construct and maintain. They can also limit the water flow into the intake, thereby limiting power production or quantity of water available. These structures can also result in impingement of fish on the intake.
Behavioral deterrents rely on fish behavior to divert fish from an intake. These deterrents include lights, electric fields, velocity/turbulence, and sound deterrents. Underwater lights (including strobes) are species-specific, and the visibility of lights, and therefore their effectiveness, is reduced in turbid water. There are three major concerns with underwater electric fields. They present a human safety concern, they are variable depending on the conductivity of water, and the fields that are effective for some fish species may be lethal to other species. Velocity/turbulence deterrents, such as air bubble curtains and waterjets, are energy intensive and don't affect some species. It is difficult to establish an uninterrupted barrier and guide with underwater sound deterrents, and long term reliability is questionable with the present generation of sound generators. Also, the frequency and amplitude of the sound wave must be tuned for each individual species, and even life stage, of fish.
Bypass structures work by releasing a certain amount of water. This discharge gives the fish water to swim in, and also creates an attraction current upstream of the bypass entrance. By discharging the water in front of the bypass, a void is created that the surrounding water fills. This movement of water toward the void creates the attraction current. This is known as a negative static head differential. One problem associated with these bypasses is that the field of attraction current is limited unless a large quantity of water is bypassed, which results in lost revenue.
There is presently limited prior art concerning the mechanical generation of currents by a velocity head differential to guide fish from hazardous areas. While migrating, fish use the water current as a guide, whether traveling upstream to spawn or downstream to the sea. As was previously mentioned, fish passage systems routinely generate currents to guide fish by a static head differential. This involves creating a situation where water falls from a higher elevation to a lower elevation. This develops an increase in water velocity and therefore a current.
Other prior art related to the mechanical generation of currents uses waterjets to create currents to guide fish. See U.S. Pat. Nos. 4,437,431 and 5,632,572. These methods are very complex, energy intensive, and more expensive than propeller generated currents.